From tagua nut - a.k.a. corozo, a tree whose nut makes durable, smooth natural buttons. Very consistent good quality, and dark navy, looks almost black depending on the light. Very smooth, well made button with rounded corners. 1/2" or 5/8"

Vegetable Ivory/Tagua/Corozo: Click here or here for wikipedia articles about this tropical palm tree nut. Tagua has been used for buttons since the 1880's, before plastic.